The present invention relates to gas bearings and more particularly concerns a bearing assembly having a floating bearing that significantly reduces required dimensional tolerances.
Various types of optical scanning devices, such as a lasar pattern generator, employ a rotating mirror from which a laser beam is reflected in successive angular sweeps across the surface of a medium upon which a pattern is to be generated or which bears a pattern that is to be read by the scanning assembly. Such systems demand extreme precision and stability of the rotating mirror because of the high tolerance requirements for positioning of the laser beam. To attain exceedingly high precision and stability for the rotating mirror position, some prior systems have employed air thrust bearings positioned between and bearing against mutually opposed bearing surfaces on the rotating part. In such systems, a thrust bearing that is fixed to a mounting plate, locates the mirror rotational shaft axis by means of a radial bearing and axially locates the shaft and mirror by a fixed length thrust bearing. The thrust bearing is positioned between mutually spaced and mutually parallel bearing surfaces on the shaft that extend in planes perpendicular to the rotation axis. Any small degree of lack of parallelism of such opposed bearing surfaces introduces significant moments on the shaft and mirror, which moments, even in very small amounts, can cause the reflected laser beam to deflect beyond acceptable limits. Such moments also introduce vibrations which are difficult to damp. The exceeding sensitivity of the system to small amounts of error in thrust surface parallelism requires tolerances in the order of a few ten thousandths of an inch, thus increasing the difficulty and greatly raising the cost of manufacturing the assembly.
Another problem with existing bearings is that air pressures must be closely adjusted and maintained to float the bearing at the designated height. The system is sensitive to loss or significant decrease in supplied air pressure. Should the air pressure supply to such a rotating mirror bearing assembly fail, or should the supplied air pressure decrease below a certain amount, the air bearing, which reacts to the load of the mirror weight (where the mirror is mounted for rotation about a vertical axis), may decrease in air film thickness to a point where the mirror is no longer supported by the air film. Resulting metal to metal contact may effectively destroy the highly precise and expensive mirror. For example, in a system arranged to operate under an eighty pounds per square inch air supply, catastrophic metal to metal contact will occur if the pressure should drop to thirty-five to forty pounds per square inch. Thus it is frequently necessary to employ a reserve air tank that will automatically supply air at the proper pressure upon a sensed decrease in pressure of the normal air supply.
It is an object of the present invention to avoid or minimize above-mentioned problems.